The present invention relates to plate-like barium ferrite particles suitable for use in magnetic recording, on which the surface thereof a solid solution of zinc is formed and a process for producing the same. The present invention relates, more particularly, to plate-like barium ferrite particles with a solid solution of zinc on the surface thereof, which have an average particle diameter of not less than 0.03 .mu.m and less than 0.1 .mu.m, a BET specific surface area of 55 to 80 m/g, a coercive force (Hc) of 300 to 1,500 Oe and a magnetization (M) of not less than 57 emu/g, preferably not less than 60 emu/g, in magnetic field of 10 KOe, and contain 2 to 13 atom % of Co with respect to Fe(III) and Ti in a molar ratio of 0&lt;Ti/Co.ltoreq.0.8, and relates to a process for producing plate-like barium ferrite particles suitable for use in magnetic recording comprising the steps of: adding 2 to 13 atom % of Co compound with respect to Fe(III) and a Ti compound in a molar ratio of 0&lt;Ti/Co.ltoreq.0.8 to an aqueous alkaline suspension of iron hydroxide (III) containing Ba ions, autoclaving the thus obtained aqueous alkaline suspension at a temperature of from 120.degree. to 330.degree. C., suspending the thus obtained plate-like barium ferrite particles containing Co and Ti in an aqueous solution containing zinc of pH 4.0 to 12.0 so as to deposit zinc hydroxides on the surface of the plate-like barium ferrite particles containing Co and Ti, filtering out and drying the particles, and then baking the thus obtained particles at a temperature of 600.degree. to 900.degree. C., to form a solid solution of zinc on the surface of the plate-like barium ferrite particles.
As described, for example, in Japanese Patent Application Laying-Open (KOKAI) No. 55-86103 (1980), ferromagnetic non-acicular particles having a large magnetization, an appropriate coercive force and an appropriate average particle size have recently been demanded as a magnetic recording material, in particular, as a perpendicular magnetic recording material.
Generally, barium ferrite particles are known as ferromagnetic non-acicular particles.
Hitherto, as a process for producing plate-like barium ferrite, a method of autoclaving an aqueous alkaline suspension containing Ba ions and Fe(III) by using an autoclave as a reaction apparatus (this method is hereinafter referred to as "autoclaving") is known.
With respect to magnetic properties, the magnetization (M) of plate-like barium ferrite particles must be as large as possible, as in described in the specification of Japanese Patent Application Laying-Open (KOKAI) No. 56-149328 (1981); ". . . the magneto plumbite ferrite which is used as a magnetic recording material is required to have the greatest possible saturation magnetization . . .".
A coercive force of about 300 to 1,500 Oe is generally required, and in order to reduce the coercive force of the produced barium ferrite particles to an appropriate coercive force in the autoclaving process, a method of substituting a part of Fe(III) in ferrite by Ti(IV) and Co(II), or Co(II) and divalent metal ions M(II) of Mn, Zn, Ni or the like has been proposed.
For example, European Patent No. 1023445 A discloses "plate-like barium ferrite particles which are represented by the formula BaCo.sub.x Ti.sub.y Fe.sub.12-x-y O.sub.19 wherein x and y independently are numbers from 0.1 to 0.5 and which have a BET specific surface area of from 20 to 70 m.sup.2 /g, an average particle diameter of from 0.05 to 0.3 .mu.m, a magnetization of larger than 30 emu/g in a magnetic field of 7.96.times.10.sup.5 A m.sup.-1 (10 KOe) and a coercive force of from 2.39.times.10.sup.4 to 7.96.times.10.sup.4 A m.sup.-1 (300 to 1,000 Oe)", and "a process for producing plate-like barium ferrite particles represented by the formula: EQU BaCo.sub.x Ti.sub.y Fe.sub.12-x-y O.sub.19
wherein x and y independently are numbers from 0.1 to 0.5, which process comprises autoclaving at a temperature of from 250.degree. to 320.degree. C. an aqueous alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions and a molar excess with respect to the total amount of Fe(III), Co(II), Ti(IV) and Ba ions of an alkali metal hydroxide, and in which suspension the atomic ratio of the total amount of Co(II) and Ti(IV) to the amount of Fe(III) is from 0.017:1 to 0.09:1 and the atomic ratio of Ba ions to the total sum of Fe(III), Co(II) and TI(IV) is from 1:7 to 1:9".
The particle size of plate-like barium ferrite particles for use in magnetic recording is required to be as fine as possible, in particular, to be not greater than 0.3 .mu.m.
This fact is described, for example, in Japanese Patent Application Laying-Open (KOKAI) No. 56-125219 (1981); ". . . perpendicular magnetic recording is effective in relation to horizontal recording in the region in which the recording wavelength is less than 1 .mu.m. In order to sufficiently record and reproduce in this wavelength region, the crystalline particle diameter of the ferrite is preferably substantially less than 0.3 .mu.m. However, if the particle diameter is about 0.01 .mu.m, the desired coercive force is not exhibited. Therefore, a crystalline particle diameter of 0.01 to 0.3 .mu.m is appropriate".
In order to reduce noise of a magnetic recording medium, it is necessary that the magnetic particle is as fine as possible and the BET specific surface area is large.
This fact is described in, for example, "FIG. 3" on lines 23 to 29, p 27, MR 81, Reports of Technical Researches of the Institute of Electronics and Communication Engineers of Japan. Namely, "FIG. 3" shows the relationship between the particle size as well as BET specific surface area and noise level of the particles in acicular maghemite particles with Co deposition. As the particle size is made smaller and the BET specific surface area increased, the noise level is linearly lowered. This relationship is the same with plate-like barium ferrite particles.
Plate-like barium ferrite particles having a large magnetization, an appropriate coercive force, an appropriate particle size and a large BET specific surface area have now been strongly demanded, but it is difficult to obtain plate-like barium ferrite particles having both large BET specific surface area and large magnetization because the magnetization of plate-like barium ferrite particles generally have a tendency to be reduced with the increase in the BET specific surface area.
The relationship between the BET specific surface area and the magnetization (M) of plate-like barium ferrite particles is obvious from "FIG. 2" and the description "FIG. 2 shows the relationship between the saturation magnetization and the specific surface area in Ba ferrite . . . larger in dimension than a single domain. . . . Saturation magnetization linearly reduces with respect to the BET specific surface area" at p 199 of the Synopses of the Lectures in 7-th Meeting of the Magnetics Society of Japan.
Actually, although Japanese Patent Application Laying-Open (KOKAI) No. 60-66321 (1985) discloses various plate-like barium ferrite particles having a particular BET specific surface area and a particular magnetization, no plate-like barium ferrite particles which are large in both BET specific surface area and magnetization, in particular, which have both a BET specific surface area of not less than 55 m.sup.2 /g and a magnetization of not less than 57 emu/g, preferably not less than 60 emu/g, have been yet obtained.
The plate-like barium ferrite particles described in the Synopses of the Lectures in 7-th Meeting of the Magnetics Society of Japan and Japanese Patent Application Laying-Open (KOKAI) No. 60-66321 (1985) are obtained by mixing a barium ferrite material with a glass forming material, melting the thus obtained mixture, and rapidly cooling the melt, namely by a so-called glass fusion process. A similar relationship between the BET specific surface area and the magnetization is found with plate-like barium ferrite particles produced by autoclaving.
The relationship between the BET specific surface area and the magnetization of the plate-like barium ferrite particles produced by autoclaving will be described in more detail as follows.
In autoclaving, different barium ferrite particles are precipitated depending upon the selected reaction condition. The precipitated particles have ordinarily a hexagonal plate-like configuration, and their particle size, BET specific surface area and magnetic properties are different depending upon the producing condition. It is therefore difficult to obtain plate-like barium ferrite particles having a BET specific surface area of not less than 55 m.sup.2 /g while maintaining the magnetization at more than 50 emu/g.
As a method of solving this problem, a method of baking plate-like barium ferrite particles produced from an aqueous solution by autoclaving at a temperature of more than 800.degree. C. to improve the magnetization is conventionally known [Japanese Patent Publication No. 60-12973 (1980)].
In this method, however, the magnetization tends to increase with the rise in baking temperature, and a high temperature of more than 900.degree. C. is required in order to obtain a large magnetization, in particular, a magnetization of not less than 57 emu/g, preferably not less than 60 emu/g. In the case of baking at more than 900.degree. C., sintering between the particles is so marked as to bulk, these formed bulk particles being unfavorable as magnetic particles for use in magnetic recording.
In order to control the coercive force (Hc) of the plate-like barium ferrite particles obtained by baking so as to be less than 1,500 Oe, a large quantity of the above-described coercive force reducing agent must be added, resulting in cause of the magnetization to be lowered. Thus, it is difficult to control the coercive force (Hc) in the range of 300to 1,500 Oe while maintaining the magnetization at a large value, in particular, at not less than 57 emu/g, preferably not less than 60 emu/g.
In autoclaving are known a method of adding zinc in the producing reaction of plate-like barium ferrite particles [for example, Japanese Patent Publication Nos. 46-3545 (1971) and 60-12973 (1985)] and a method of coating the surfaces of plate-like barium ferrite particles with the oxide and/or the hydroxide of zinc [Japanese Patent Application Laying-Open (KOKAI) No. 58-56232 (1983)]. By any of these methods, it is difficult to obtain plate-like barium ferrite particles having a large magnetization (M) and a coercive force (Hc) of 300 to 1,500 Oe.
Thus, the offer of plate-like barium ferrite particles having a large magnetization, an appropriate coercive force and an appropriate average particle size as a magnetic material for magnetic recording, in particular, as a perpendicular magnetic recording material have been strongly demanded.
As a result of various studies on a process for producing plate-like barium ferrite particles having a large BET specific surface area, a large magnetization and an average particle size of less than 0.1 .mu.m by autoclaving, it has been found that plate-like barium ferrite particles having a BET specific surface area of not less than 55 m.sup.2 /g, a magnetization (M) of not less than 57 emu/g, preferably not less than 60 emu/g and a coercive force (Hc) of 300 to 1,500 Oe by suspending plate-like barium ferrite particles containing Co and Ti, the atomic ratio of Co to Fe(III) being from 0.02:1 to 0.13:1 and the molar ratio of Ti to Co being from more than 0 to not more than 0.8, in an aqueous solution containing zinc of pH 4.0 to 12.0 to obtain the plate-like barium ferrite particles deposited with the zinc hydroxide on the surfaces thereof, filtering out and drying the particles, and thereafter baking the particles at a temperature of 600.degree. to 900.degree. C. to form a solid solution of zinc on the surfaces of the plate-like barium ferrite particles, and based on the finding the present invention has been attained.